1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly, the present invention relates to an in-plane switching mode liquid crystal display devices in which bad contacts of common lines are prevented and a fabricating method thereof.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device makes use of the optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational alignment that results from their thin and long shape. The alignment direction of the liquid crystal molecules can be controlled by application of an electric field to the liquid crystal molecules. Accordingly, as an intensity of the applied electric field changes, the alignment orientation of the liquid crystal molecules also changes. Because incident light through a liquid crystal material is refracted due to an orientation of the liquid crystal molecules resulting from the optical anisotropy of the aligned liquid crystal molecules, an intensity of the incident light can be controlled and images can be displayed.
Among the various types of LCD devices commonly used, active matrix LCD (AM-LCD) devices, in which thin film transistors (TFTs) and pixel electrodes connected to the TFTs are disposed in a matrix, have been developed because of their high resolution and superior display of moving images.
The LCD device includes upper and lower substrates, and a liquid crystal layer interposed therebetween. The upper substrate, which is referred to as a color filter substrate, has a common electrode and the lower substrate, which is referred to as an array substrate, has a pixel electrode. The liquid crystal layer is driven with an electric field generated between the common electrode and the pixel electrode. The LCD device having the common electrode and the pixel electrode on opposite substrates has excellent transmittance and aperture ratio. However, since the electric field is generated perpendicular to the upper and lower substrates, the LCD device has a poor viewing angle property. To solve the problem of narrow viewing angle, new LCD devices such as an in-plane switching (IPS) mode LCD device, where an electric field is laterally generated, may be used.
FIG. 1 is a schematic cross-sectional view of an in-plane switching mode liquid crystal display device according to the related art.
In FIG. 1, an upper substrate (a color filter substrate) 10 and a lower substrate 20 (an array substrate) face into and are spaced apart from each other. A liquid crystal layer 12 is interposed between the upper and lower substrates 10 and 20. A pixel electrode 36 and a common electrode 38 are formed on an inner surface of the lower substrate 20. The liquid crystal layer 12 is driven with a horizontal electric field generated between the pixel electrode 36 and the common electrode 38.
FIGS. 2A and 2B are schematic cross-sectional views showing an OFF state and an ON state of an in-plane switching mode liquid crystal display device according to the related art, respectively.
In FIG. 2A, because a horizontal electric field is not generated in the OFF state, liquid crystal molecules 13 are not re-aligned. Thus, a phase transition of a liquid crystal layer 12 does not occur.
In FIG. 2B, voltages are applied to a pixel electrode 36 and a common electrode 38, thereby an electric field 21 having horizontal and vertical portions is generated. In the vertical portion over the pixel electrode 36 and the common electrode 38, first liquid crystal molecules 13a of the liquid crystal layer 12 are not re-aligned and a phase transition of the liquid crystal layer 12 does not occur. In the horizontal portion between the pixel electrode 36 and the common electrode 38, second liquid crystal molecules 13b of the liquid crystal layer 12 are horizontally re-aligned along the electric field 21 generated by applying voltages to the pixel electrode 36 and the common electrode 38. Thus, a phase transition of the liquid crystal layer 12 occurs in the horizontal portion. Because the liquid crystal molecules are re-aligned along a horizontal electric field, the IPS mode LCD device has a wide viewing angle. For example, users can see images having a respective viewing angle of about 80° to about 85° along top, bottom, right and left direction with respect to a normal direction of the IPS mode LCD device.
FIG. 3 is a schematic plane view showing an array substrate for an in-plane switching mode liquid crystal display device according to the related art.
In FIG. 3, a plurality of gate lines 52, a plurality of common lines 54 and a plurality of data lines 56 are formed on an array substrate 50. A gate pad 51 is formed at one end of each gate line 52. The common line 54 is parallel to and spaced apart from the gate line 52. A data pad 57 is formed at one end of each data line 56. The data line 56 crosses the gate line 52 and the common line 54. A pixel region “P” is defined by the crossing of the gate line 52 and the data line 56. A gate pad terminal 80 and a data pad terminal 82 are formed on the gate pad 51 and the data pad 57, respectively. One end of each common line 54 is connected to a terminal line 66 supplying a common voltage through a connection electrode 68, and the other ends of the plurality of common lines 54 are combined. A thin film transistor (TFT) “T” including a gate electrode 58, an active layer 60 and source and drain electrodes 62 and 64 is formed adjacent the crossing of the gate line 52 and the data line 56. The source electrode 62 is connected to the data line 56 and the gate electrode 58 is connected to the gate line 52.
A pixel electrode 70 connected to the drain electrode 64 is formed in the pixel region “P.” A common electrode 72 parallel to the pixel electrode 70 and connected to the common line 56 is also formed in the pixel region “P.” The pixel electrode 70 includes a first pixel portion 70a over the gate line 52 and a second pixel portion 70b extending from the first pixel portion 70a to the pixel region “P.” The second pixel portion 70b is connected to the drain electrode 64. The common electrode 72 includes a first common portion 72a over the common line 54, and second and third common portions 72b and 72c extending from the first common portion 72a. The first common portion 72a is connected to the common line 54. The second common portion 72b is formed in an upper half of the pixel region “P” and the third common portion 72c is formed in a lower half of the pixel region “P.” The common electrode 72 is made of a transparent metallic material and the pixel electrode 70 is made of the same material as the source and drain electrodes 62 and 64. A storage capacitor “C” uses a portion of the gate line 52 as a first capacitor electrode and the first pixel portion 70a as a second capacitor electrode.
A common voltage is applied to the common electrode 72 through the common line 54. To apply a constant common voltage to the entire substrate 50 continuously, one end of each common line 56 is connected to the terminal line 66 and the other end of the plurality of common lines 56 are combined. Because the common line 56 is covered with an insulating layer, a resistance-measuring method can not be used even when a bad contact occurs between the common line 54 and the terminal line 66. Accordingly, defects such as a bad contact are detected through an electric inspection after the fabricating process of the array substrate is completed. Because the array substrate having defects can not be used for the LCD device, production yield is reduced and fabricating cost increases.